The present invention relates to a method for determining interactions between a number of optical channels during optical wavelength division multiplexed (WDM) transmission.
Non-linear interference occurs when transmitting WDM signals through optical fibers. In an optical transmission system, these effects must be determined and, in addition, must be minimized. Methods to measure the quality characteristics of a signal have been developed for this purpose.
Normal methods for measuring the non-linear effects in WDM transmission systems are based on determining a quality parameter such as the Q factor or the bit error rate (see xe2x80x9cOptical Fiber Communicationsxe2x80x9d, IIIA, I. P. Kaminow, T. L. Koch, p. 316); for example, via a decision stage. This results in a criterion being determined for the signal quality and/or for the quality of all the channels in the signal.
One such measurement method is known from Patent Application DE 199 14 793 A1, which describes a method and an arrangement for measuring the signal quality in an optical transmission system. A received binary signal BS is sampled using different thresholds, and the sampling results are integrated and stored. The measured probability distribution and probability density distribution can be used to draw conclusions relating to the signal quality; for example, the bit error rate (Abstract). FIG. 13 in this reference illustrates a measurement arrangement for WDM signals, in which different quality parameters are determined for each channel in a WDM signal.
The following illustrations describe a number of variants of the process of measuring the quality of an optical transmitted WDM signal according to the prior art, whose characteristics are explained.
FIG. 1 shows a conventional implementation of a measurement device for determining the Q factor or the bit error rate BER of a WDM signal S transmitted via an optical fiber LWL. The measurement can be carried out at the end of the transmission path LWL and at a point in the transmission fiber LWL via an inputting device EE. The WDM signal S is fed into a wavelength division demultiplexer F, such as a tunable filter, so that all the channels are selectively separated. Each channel is then supplied to a receiver Rx for quality measurement. The receiver Rx produces, for example, the values of the Q factor Q or of the bit error rate BER for all the channels.
FIG. 2 shows an exemplary embodiment of a receiver Rx for determining the bit error rate BER and for deriving the Q factor Q, which can be done from this, for one channel. A channel signal K selected from a WDM signal is supplied to a decision stage ES1 with a fixed decision threshold V1. The channel signal K is also sampled, using a variable decision threshold V2, in a decision stage ES2 arranged in parallel with the decision stage ES1. The output signals from the decision stages ES1 and ES2 are supplied to a logic xe2x80x9cexclusive ORxe2x80x9d element XOR. Different logic states are determined at the output of the element XOR and are counted by an error counter EC. The count results for different threshold values are used to determine the associated Q factors Q or bit error rates BER for one channel. The Q factor also can be derived directly from any desired recorded amplitude histograms.
FIG. 3 shows the profile of the Q factor Q for two different transmission paths as a function of the variable decision threshold V2. Optimum sampling results in a high Q factor Q, such as Q greater than 6.5, or a measured bit error rate BER which is as low as possible, such as BER less than 10xe2x88x9210.
For transmission which is protected by an error correcting code FEC (forward error correction), the bit error rate BER can be established from the number of corrected bits. An exemplary embodiment with FEC is described in FIG. 1 in the prior Patent Application P 01103107.7.
Two different effects during the transmission of wavelength division multiplexed (WDM) signals via an optical fiber may be the governing factors for non-linear interference as the reason for interactions between the channels, so-called multiple channel interactions. These are the Kerr effect and the non-linear scattering process. The Kerr effect mainly causes four wave mixing (FWM) and/or cross phase modulation (XPM). The non-linear scattering process results in stimulated Raman scattering (SRS). These non-linear effects are described in detail in the reference xe2x80x9cFiber-Optic Communication Systemsxe2x80x9d, G. P. Agrawal, 2nd Edition, pp. 323-328. These multiple channel interactions occur, in particular, in the case of xe2x80x9cDense Wavelength Division Multiplexxe2x80x9d (DWDM) transmission, because the channel separations are even less than in WDM systems. For optimum transmission, these effects must be measured and minimized for the Q factor Q to have a constant value, which is as high as possible, for all channels.
An object of the present invention is, thus, to specify a method for determining and distinguishing between multiple channel interactions, which are caused, in particular, by Kerr effects and scattering processes, during the transmission of a WDM signal.
According to the present invention, therefore, the multiple channel interactions are distinguished by determining and evaluating the spectral profile of the Q factor along the bandwidth of a WDM or DWDM signal. This is done by sampling all the channels in the WDM signal, in which case it is possible to determine an amplitude histogram for each channel, in order to calculate the Q factor. The correction statistics can be used for protected data signals.
A comparison, carried out via an evaluation, between the Q factor values of the channels at the boundary and in the center of the bandwidth provides a conclusion as to whether the governing factor causing multiple channel interactions is Kerr effects or a non-linear scattering process.
In the case of DWDM transmission with a large number of channels with narrow frequency separations, the determination of the Q factor can be restricted to a number of channels at the boundary or in the center of the bandwidth. If there are a large number of channels, it is worthwhile recording statistical variations in the measurement results.
If the measurement of the Q factor is being influenced by high dispersion, this is compensated for, first of all, in the same way as other interference effects.
The WDM system can be optimized on the basis of the measurements of the profile of the Q factor.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the Figures.